Orbital motion of gold heterodimer driven by optical force of circularly polarized light and reactive drag of medium

This theoretical study explores the two-dimensional orbital motion of an optically bound heterodimer consisting of two gold nanoparticles ( NPs) of different sizes, driven by circularly polarized (CP) light. Although a CP light possesses only spin angular momentum without orbital angular momentum, it can still induce orbital revolution in the plasmonic heterodimer. This phenomenon arises from the interaction between the optical force and torque generated by the CP light and the reactive drag force and torque from the surrounding medium. We calculate the optical forces acting on each NP by analyzing the Maxwell stress tensor at their surfaces, and we account for the reactive drag force using Stokes' law. These forces are used to simulate the trajectories of the NPs through dynamic equations of motion. Our results demonstrate that, regardless of the initial conditions of the two NPs, they will become optically bound together, exhibiting rigid-body translation and rotation. Notably, the center of mass of the heterodimer undergoes an orbital revolution around a fixed point eventually. The CP light-manipulated heterodimer behaves like a boomerang, acting as a spinning rotor on a circular path. The heterodimer's orbital radius and direction of revolution are influenced by the size disparity between the two NPs. Additionally, each NP experiences spin motion, with the spin direction determined by the handedness of the CP light. The optically bound gold heterodimer functions as a light-driven microrotor, with potential applications in microfluidic channels. These findings offer valuable insights into the optomechanical manipulation of non-monodisperse NP clusters using CP light.